1. Field of the Invention
The present invention relates to a display device having a pixel circuit (referred to also as a pixel) provided with an electrooptic element (referred to also as a display element or a light emitting element), and particularly to a display device having a current-driven type electrooptic element changing in luminance according to the magnitude of a driving signal as a display element, and having an active element in each pixel circuit, display driving being performed in a pixel unit by the active element.
2. Description of the Related Art
There are display devices that use an electrooptic element changing in luminance according to a voltage applied to the electrooptic element or a current flowing through the electrooptic element as a display element of a pixel. For example, a liquid crystal display element is a typical example of an electrooptic element that changes in luminance according to a voltage applied to the electrooptic element, and an organic electroluminescence (hereinafter described as organic EL) element (organic light emitting diode (OLED)) is a typical example of an electrooptic element that changes in luminance according to a current flowing through the electrooptic element. An organic EL display device using the latter organic EL element is a so-called emissive display device using a self-luminous electrooptic element as a display element of a pixel.
The organic EL element includes an organic thin film (organic layer) formed by laminating an organic hole transporting layer and an organic light emitting layer between a lower electrode and an upper electrode. The organic EL element is an electrooptic element using a phenomenon of light emission occurring on application of an electric field to the organic thin film. A color gradation is obtained by controlling the value of current flowing through the organic EL element.
The organic EL element can be driven by a relatively low application voltage (for example 10 V or lower), and thus consumes low power. In addition, the organic EL element is a self-luminous element that emits light by itself, and therefore obviates a need for an auxiliary illuminating member such as a backlight desired in a liquid crystal display device. Thus the organic EL element facilitates reduction in weight and thickness. Further, the organic EL element has a very high response speed (for example a few μs or so), so that no afterimage occurs at a time of displaying a moving image. Because the organic EL element has these advantages, flat-panel emissive display devices using the organic EL element as an electrooptic element have recently been actively developed.
Display devices using an electrooptic element including liquid crystal display devices using a liquid crystal display element and organic EL display devices using an organic EL element can adopt a simple (passive) matrix system and an active matrix system as a driving system of the display devices. However, while having a simple structure, a simple matrix type display device presents for example a problem of difficulty in realizing a large and high-definition display device.
Thus an active matrix system that controls a pixel signal supplied to a light emitting element within a pixel by using an active element similarly provided within the pixel, for example an insulated gate field effect transistor (typically a thin film transistor (TFT)) as a switching transistor has recently been actively developed.
When an electrooptic element within a pixel circuit is made to emit light, an input image signal supplied via a video signal line is captured into a storage capacitor (referred to also as a pixel capacitance) provided to the gate terminal (control input terminal) of a driving transistor by a switching transistor (referred to as a sampling transistor), and a driving signal corresponding to the captured input image signal is supplied to the electrooptic element.
In a liquid crystal display device using a liquid crystal display element as an electrooptic element, because the liquid crystal display element is a voltage-driven type element, the liquid crystal display element is driven by a voltage signal itself corresponding to an input image signal captured into a storage capacitor. On the other hand, in an organic EL display device using a current-driven type element such as an organic EL element or the like as an electrooptic element, a driving transistor converts a driving signal (voltage signal) corresponding to an input image signal captured into a storage capacitor into a current signal, and the driving current is supplied to the organic EL element or the like.
The current-driven type electrooptic element typified by the organic EL element varies in light emission luminance when the value of the driving current varies. Hence, in order to make the electrooptic element emit light at stable luminance, it is important to supply stable driving current to the electrooptic element. For example, a driving system for supplying the driving current to the organic EL element can be roughly classified into a constant-current driving system and a constant-voltage driving system (which are well known techniques, so that publicly known documents will not be presented here).
Because the voltage-current characteristic of the organic EL element has a steep slope, when constant-voltage driving is performed, slight variations in voltage or variations in element characteristic cause great variations in current and thus bring about great variations in luminance. Hence, constant-current driving in which the driving transistor is used in a saturation region is generally used. Of course, even with constant-current driving, changes in current invite variations in luminance. However, small variations in current cause only small variations in luminance.
Conversely, even with the constant-current driving system, in order for the light emission luminance of the electrooptic element to be unchanged, it is important for the driving signal written to the storage capacitor according to the input image signal and retained by the storage capacitor to be constant. For example, in order for the light emission luminance of the organic EL element to be unchanged, it is important for the driving current corresponding to the input image signal to be constant.
However, the threshold voltage and mobility of the active element (driving transistor) driving the electrooptic element vary due to process variations. In addition, characteristics of the electrooptic element such as the organic EL element or the like vary with time. Such variations in the characteristics of the active element for driving and such variations in the characteristics of the electrooptic element affect light emission luminance even in the case of the constant-current driving system.
Thus, various mechanisms for correcting luminance variations caused by the above-described variations in the characteristics of the active element for driving and the electrooptic element within each pixel circuit are being studied to uniformly control the light emission luminance over the entire screen of a display device.
For example, a mechanism described in Japanese Patent Laid-Open No. 2006-215213 (hereinafter referred to as Patent Document 1) as a pixel circuit for an organic EL element has a threshold value correcting function for holding driving current constant even when there is a variation or a secular change in threshold voltage of a driving transistor, a mobility correcting function for holding the driving current constant even when there is a variation or a secular change in mobility of the driving transistor, and a bootstrap function for holding the driving current constant even when there is a secular change in current-voltage characteristic of the organic EL element.
During threshold value correcting operation, a power supply voltage of a predetermined magnitude is supplied to the power supply terminal of the driving transistor to create a state of a current flowing between the drain and the source of the driving transistor, and the sampling transistor is made to conduct with a reference potential of a predetermined magnitude for threshold value correction supplied to the input terminal of the sampling transistor.
In this case, depending on driving timing, the period of threshold value correcting operation may be insufficient, and thus a voltage corresponding to the threshold voltage of the driving transistor may not be completely retained in the storage capacitor. For a measure against such a phenomenon, adopting a mechanism of making the storage capacitor surely retain the voltage corresponding to the threshold voltage of the driving transistor by repeatedly performing threshold value correcting operation a plurality of times is considered (see Japanese Patent Laid-Open No. 2005-258326).